Digital cameras for emitting auxiliary light (flash light) to subjects and taking images are in widespread use.
In these digital cameras, as a scheme of controlling an amount of auxiliary light emission when image taking is performed, a preliminary light emission control scheme is known.
The preliminary light emission control scheme is a scheme of sequentially performing non-light emission image taking in which auxiliary light emission is not performed, and pre-light emission image taking in which preliminary light emission (pre-light emission) is performed such that a predetermined amount of auxiliary light is emitted, prior to primary image taking, and calculating an amount of auxiliary light emission for primary image taking, on the basis of a brightness difference between an image taken by the non-light emission image taking and an image taken by the pre-light emission image taking.
In the preliminary light emission control scheme, specifically, an amount of light emission for primary image taking is obtained as follows, for example.
First, the image taken by the non-light emission image taking and the image taken by the pre-light emission image taking are divided into a plurality of areas common to them, and for each area, a brightness difference between the image taken by the non-light emission image taking and the image taken by the pre-light emission image taking is calculated. Next, the brightness difference calculated for each area is multiplied by a weight coefficient set for each area, whereby an evaluation value is calculated for each area. Next, an area with a large evaluation value is selected as a main subject area, and an amount of light emission for primary image taking is determined such that the main subject area has appropriate brightness.
In this preliminary light emission control scheme, in a scene where a light source exists in the background of a main subject, in a case where the light source has moved during pre-light emission image taking, a case where the position of the light source in the taken image has changed by shakes of a digital camera, and the like, it is impossible to perform satisfactory image taking.
In the case where the light source has moved during the pre-light emission image taking, and the case where the position of the light source in the taken image has changed by shakes of a digital camera, a brightness difference of an area including the light source in an image taken by the pre-light emission image taking becomes large. As a result, the evaluation value of that area becomes large, and thus the area including the light source is erroneously selected as a main subject area. If this erroneous selection exists, an amount of light emission for primary image taking is dragged by the light source of the background of the main subject such that the amount of light emission is set to be small, and thus it is impossible to perform satisfactory image taking.
Patent Document 1 discloses a camera using the preliminary light emission control scheme and a so-called flash-matic scheme of determining an amount of light emission on the basis of a distance to a subject, a diaphragm of an imaging optical system, ISO sensitivity, and so on. In this camera, a subject state is determined by comparing an amount of light emission determined by the preliminary light emission control scheme with an amount of light emission determined by the flash-matic scheme, and according to the determined subject state, which one of the amounts of light emission determined by those schemes will be used as an amount of light emission for image taking is determined.
According to the camera disclosed in Patent Document 1, since an amount of light emission determined by any one of the two dimming schemes is set as an amount of auxiliary light emission for primary image taking, it is possible to accurately determine the amount of auxiliary light emission for primary image taking.    Patent Document 1: Japanese Patent Application Publication No.: 2004-264783
However, in the camera disclosed in Patent Document 1, in the preliminary light emission control scheme, there is a possibility that a reduction in the amount of light emission according to movement of a light source as described above will occur. In a case where a reduction in the amount of light emission occurs, it is impossible to accurately determine a subject state, and as a result, it is impossible to accurately determine control on the amount of light emission. Also, since it is necessary to perform both of calculation of the amount of light emission according to the preliminary light emission control scheme and calculation of the amount of light emission according to the flash-matic scheme, it takes a long time to determine the amount of light emission.
The present invention was made in view of the above-mentioned circumferences, and an object of the present invention is to provide an imaging device capable of quickly determining an amount of auxiliary light emission with high accuracy, and a light emission amount control method of the imaging device.